Once a petroleum well has been drilled and cased, it is often necessary or desired to drill one or more additional wells that branch off, or deviate, from the first well. Such multilateral wells are typically directed toward different parts of the surrounding formation, with the intent of increasing the output of the well. The main well bore can be vertical, angled or horizontal. Multilateral technology can be applied to both new and existing wells.
In order to drill a new borehole that extends outside an existing cased wellbore, the usual practice is to use a work string to run and set a whipstock via an anchor disposed at the lower end thereof. The upper end of the whipstock comprises an inclined face. The inclined face is designed to guide a window milling bit radially outwardly with respect to the casing axis as the milling bit is lowered, so that the milling bit engages and cuts a window in the casing. The lower end of the whipstock is adapted to engage an anchor in a locking manner that prevents both axial and rotational movement.
Multilateral technology provides operators several benefits and economic advantages. For example, multilateral technology can allow isolated pockets of hydrocarbons, which might otherwise be left in the ground, to be tapped. In addition, multilateral technology allows the improvement of reservoir drainage, increasing the volume of recoverable reserves and enhancing the economics of marginal pay zones. By utilizing multilateral technology, multiple reservoirs can be drained simultaneously. Thin production intervals that might be uneconomical to produce alone become economical when produced together with multilateral technology. Multiple completions from one well bore also facilitate heavy oil drainage.
In addition to production cost savings, development costs also decrease through the use of existing infrastructure such as surface equipment and the well bore. Multilateral technology expands platform capabilities where slots are limited and eliminates spacing problems by allowing more drain holes to be added within a reservoir. In addition, by sidetracking damaged formations or completions, the life of existing wells can be extended. Laterals may be drilled below a problem area once casing has been set, thereby reducing the risk of drilling through troubled zones. Finally, multilateral completions accommodate more wells with fewer footprints, making them ideal for environmentally sensitive or challenging areas.
Often however, a well bore is configured such that a tubular string of a smaller diameter is contained within a larger pipe string, casing, or open hole, thereby making it necessary to run well tools through the smaller diameter tubular and thereafter perform downhole operations (such as sidetracking) within the larger area provide by the larger pipe string, casing, or open hole. Expandable tools are generally used for such operations. Disadvantages of known expandable anchors include limited radial expansion capabilities and limited capability of securing the anchor within the larger diameter. As such, prior expandable anchors that support whipstocks for drilling sidetrack boreholes, for example, may be susceptible to small but not insignificant amounts of movement. Hence, it would be desirable to provide an expandable anchor that effectively prevents an anchored whipstock from moving.